This invention relates to an automatic balancing mechanism and, more particularly, to an automatic balancing mechanism for a disk driver free from vibrations due to a characteristic angular velocity of a rotor.
A driving mechanism is, by way of example, incorporated in an optical disk memory system, and an optical disk is driven for rotation by the driving mechanism.
A typical example of the disk driving mechanism is illustrated in FIG. 1 of the drawings. An electric motor 1 has a rotor 2, and the rotor 2 is rotated around an axis 3 of rotation. A turn table 4 is fixed to the rotor 2, and is driven for rotation by the electric motor 1. A pulley 5 is movable in the direction of axis 3, and magnetic force or elastic force of a spring is exerted on the pulley 5, and presses an optical disk 6 against the turn table 4. While the rotor 2 is turning around the axis 3, the turn table 4, the pulley 5 and the optical disk 6 turn around the axis 3 together with the rotor 2. Thus, the turn table 4, the pulley 5 and the optical disk 6 turn together, and assume to have a center of gravity. If the center of gravity is aligned with the axis 3, any unbalance does not take place, and the turn table 4, the pulley 5 and the optical disk 6 are stable during the rotation. However, the turn table 4, the pulley 5 and the optical disk 6 are assembled and disassembled at every usage, and it is impossible to make the center of gravity aligned with the axis 3 at all times. Thus, the unbalance is unavoidable, and is causative of vibrations during high-speed rotation. The magnitude of vibrations is dependent on the amount of unbalance and the rotating speed. The distance between the axis 3 and the center of gravity and the weight of the assembly 4/5/6 affect the amount of unbalance.
The disk driving mechanism has been designed to rotate the disk at relatively low speed, and the centrifugal force due to the unbalance is relatively small. Even though the optical disk 6 vibrates due to the unbalance of the assembly 4/5/6, a data read-out head (not shown) exactly reads out data bits from the optical disk. For this reason, any anti-vibration means is not provided for the prior art disk driving mechanism.
Data access speed is getting faster and faster, and a constant linear velocity disk driving mechanism drives the optical disk at 6000 rpm during a data access to an inside area. When the optical disk is driven for rotation at more than 4000 rpm, the vibrations due to the unbalance becomes serious, and the read-out head falls into an error in the data read-out. Thus, a suitable anti-vibration means is required for the high-speed disk driving mechanism.
An automatic balancing mechanism is well known in the field of mechanical dynamics. For example, an automatic balancing mechanism is introduced in the book entitled as xe2x80x9cMechanical Dynamicsxe2x80x9d, and FIG. 2 illustrates the automatic balancing mechanism. A circular groove 10 is formed in a disk 11 integral with a rotor, and two balls 12/13 are movable along the circular groove 10. When the disk 11 is driven for rotation, the centrifugal force F is exerted on each ball 12/13, and is given by equation 1.
F=m r xcfx892 xe2x80x83xe2x80x83Equation 1 
Where m is the mass of the ball 12/13, r is the radius of curvature of the circular groove and xcfx89 is the angular velocity. The component force F1 in X direction and the component force F2 in Y direction are expressed by equations 2 and 3.
F1=m r xcfx892 sin xcex1xe2x80x83xe2x80x83Equation 2 
F2=m r xcfx892 cos xcex1xe2x80x83xe2x80x83Equation 3 
where xcex1 is the angle between X-axis and the line drawn between the ball 12/13 and the center S of the disk 11. If the center of gravity G of the rotor is deviated from the center S of the disk 11 by distance e, unbalance takes place, and the centrifugal force F3 due to the unbalance is given by equation 4.
F3=Mexcfx892 xe2x80x83xe2x80x83Equation 4 
where M is the mass of the rotor. If the centrifugal forces F are balanced with the centrifugal force F3, the balance in Y-direction is expressed as
F1+(xe2x88x92F1)=0 xe2x80x83xe2x80x83Equation 5 
The component force F1 exerted on the ball 12 cancels the component force F1 exerted on the other ball 13. On the other hand, the component forces F2 exerted on the balls 12/13 are balanced with force F3, and the balance in X-direction is expressed as
m r xcfx892 cos xcex1=Mexcfx892 xe2x80x83xe2x80x83Equation 6 
Therefore, the balls 12/12 are positioned at certain positions satisfying equation 6.
When the automatic balancing mechanism 11/12/13 is simply used for the prior art disk driving mechanism 1/4/5/6 incorporated in the optical data storage system, serious vibrations suddenly take place in the assembly 4/5/6. The serious vibrations are derived from the characteristic angular velocity as follows. FIGS. 3A and 3B illustrate two kinds of relative relation between the center of gravity G and the balls 12/13. Point xe2x80x9cOxe2x80x9d is indicative of the center of bearings supporting the rotor, and the component force N in the normal direction and the component force T of the tangential direction form the centrifugal force F.
If the angular velocity xcfx89 is less than the characteristic angular velocity xcfx89 0, the center of gravity G is on the same side as the balls 12/13 with respect to the center S as shown in FIG. 3A. In this situation, the component forces N are balanced with the reaction from the disk 11, and the component forces T make the balls 12/13 closer to each other. Then, the unbalance takes place, and is increased together with the positions of the balls 12/13.
On the other hand, if the angular velocity xcfx89 is greater than the characteristic angular velocity xcfx890, the center of gravity G is on the line between the center S and the center O as shown in FIG. 3B. In this situation, the component forces N are also balanced with the reaction from the disk 11, and the component forces T cause the balls 12/13 to place at the appropriate positions shown in FIG. 2. Then, the center S is coincident with the center O, and the component forces T are decreased to zero.
When the electric motor 1 is energized, the electric motor 1 increases the angular velocity xcfx89, and the angular velocity xcfx89 exceeds over the characteristic angular velocity xcfx890. While the electric motor 1 is increasing the angular velocity xcfx89 under the characteristic angular velocity xcfx890, the automatic balancing mechanism 11/12/13 and the prior art disk driving mechanism 1/4/5/6/ are established in the relative relation shown in FIG. 3A, and the serious vibrations take place. Even after the angular velocity xcfx89 exceeds over the characteristic angular velocity xcfx890, there is a possibility to move the balls 12/13 from the positions shown in FIG. 3B to the positions shown in FIG. 3A, and the movement causes the serious vibrations to take place.
It is therefore an important object of the present invention to provide an automatic balancing mechanism which prevents a driving mechanism from vibrations due to the characteristic angular velocity.
To accomplish the object, the present invention proposes to forcibly locate the center of gravity at a certain point opposite to movable weight members.
In accordance with one aspect of the present invention, there is provided an automatic balancing mechanism associated with a rotor driven for rotation around a rotating axis comprising a first weight means associated with the rotor so as to make a center of gravity offset from the rotating axis of the rotor, a stopper means stationary with respect to the rotor and defining a first moving path on the opposite side to the first weight means and the center of gravity with respect to a virtual line perpendicular to the rotating axis, and a plurality of second weight means equal in number to a multiple of two and moved to respective balancing positions on the first moving path due to centrifugal force exerted thereon during a rotation of the rotor so as to cancel unbalance due to the center of gravity.